Parylene is a generic term applied to a class of poly-p-xylylenes which includes those derived from the chlorinated dimer of the structure: ##STR1## Chlorinated di-p-xylylene, hereinafter also referred to as dichloro-[2,2]paracyclophane, upon pyrolysis followed by polymerization forms an inert, transparent, conformal coating which has excellent barrier properties and can be used at relatively high temperatures. Due to its ability to provide thin films and conform to substrates of varied geometric shapes, it is ideally suited for use as a conformal coating in a wide variety of fields, particularly in the electronics industry.
Prior to the present invention, dichloro-[2,2]paracyclophane was only prepared by the chlorination of 2,2-paracyclophane in accordance with known procedures. For example, in U.S. Pat. No. 3,221,068 which issued Nov. 30, 1965 to William F. Gorham and is assigned to Union Carbide Corporation, there is disclosed a process for halogenating 2,2-paracyclophane to form a product which can be used as the starting material in the preparation of conformal coatings.
In the process described in the Gorham patent substituted cyclic dimers were prepared by halogenating a cyclic di-p-xylylene with free chlorine or bromine to provide a dimer with as many as six halogen substituents on the aromatic rings. The process employed a Friedel-Crafts catalyst and a temperature between about 0.degree. and 80.degree. C. Dichloro- to hexachloro- substituted dimers are disclosed and claimed as well as the dibromo derivatives. Yields of the dichloro-di-p-xylylene as high as 99 percent are disclosed in Example 1 of the patent.
Although the process disclosed in U.S. Pat. No. 3,221,068 provides dichloro-[2,2]paracyclophane, it is not without its disadvantages. For instance, direct chlorination of 2,2-paracyclophane, results in a mixture of various amounts of chlorine atoms on the benzene rings since there are eight ring sites available in addition to the less reactive alpha sites. In practice, it has been found that the chlorination of 2,2-paracyclophane must be effected under very carefully controlled conditions if the desired dichloro derivative is to be obtained. Moreover, even with such controls, the average number of chlorine atoms per dimer molecule is slightly more than two. Hence, the product obtained is not entirely pure dichloro-[2,2]paracyclophane but contains residues of other isomers.
Additionally, while the yield of chlorinated product based on the dimer obtained by the process disclosed in the patent is high, the overall yield of dichloro-[2,2]paracyclophane, based on the p-methylbenzyltrimethylammonium salt is on average no greater than 50 percent. This is due to the fact that known processes for preparing the unsubstituted dimer itself give only low yields. Yields as low as 17-19 percent are not uncommon. Accordingly, the overall process for the application of parylene as a conformal coating is expensive and severely restricts its application to other areas where it might otherwise be used if the economics were more attractive.
More recently, U.S. Pat. No. 4,532,369, issued on July 30, 1985 to Hartmut Hartner of the Federal Republic of Germany, discloses and claims a process for the preparation of 2,2-paracyclophane from p-methylbenzyltrimethylammonium hydroxide. It is indicated in the patent that known processes which existed prior to the invention disclosed therein gave only low yields or the starting materials were not readily accessible. By contacting aqueous p-methylbenzyltrimethylammonium hydroxide with sodium or potassium hydroxide in the presence of dimethylsulfoxide (DMSO) the patentee indicated that yields as high as 70 percent were obtained. It was also indicated at column 1, lines 55-58, that the resulting high yields were surprising since the addition of other comparably aprotic solvents such as dimethylformamide, N-methyl-pyrollidone or sulfolane had no effect.
However, even though the desired 2,2-paracyclophane was obtained in good yields, it was still necessary to conduct a further, direct chlorination step, which did not avoid the problem of isomer formation.
The presence of halogenated isomers other than the desired dichloro-[2,2]paracyclophane are undesireable since it is known that alphachlorinated impurities dehalogenate under conditions employed in the deposition of parylene, causing higher than normal system pressures and possible corrosion. Moreover, conformal films prepared from chlorinated paracyclophanes, which have alpha-chlorinated impurities are inferior and lose their desirable properties more rapidly at elevated temperatures.
It was therefore desirable to find a method whereby the dichloro-[2,2]paracyclophane could be synthesized in high yield without the need for a separate chlorination step and which would avoid the formation of undesireable chlorinated isomers.
It is therefore an object of this invention to provide an improved process for the preparation of dichloro-[2,2]paracyclophane useful in the formation of parylene. Another object of the invention is to provide a process for the preparation of dichloro-[2,2]paracyclophane in which undesired isomers are absent. A still further object of this invention is to provide a process for the preparation of the chlorinated dimer in relative high yields as opposed to the methods disclosed in the literature to date. Another object of the invention is to provide a process which is simple and efficient and hence is effective in reducing the overall cost in the preparation of dichloro-[2,2]paracyclophane by circumventing the need to prepare 2,2-paracyclophane and then subjecting it to the chlorination step. It is also an object of this invention to provide a process for the preparation of dichloro[2,2]paracyclophane in high yields by means of careful control of reaction temperature and temperature profile, reaction feed configuration, water concentration in the reactor, and the use of a cosolvent and/or certain reaction promoters. Another object of this invention is to provide a process for the preparation of dichlor[2,2]paracyclophane more efficiently and in greater yields than heretofore. A still further object of the present invention is to provide certain novel intermediate salts and a dichloro-[2,2]paracyclophane which is free of undesired isomers. These and other objects will readily become apparant to those skilled in the art in the light of the teachings herein set forth.